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https://issues.apache.org/jira/browse/SPARK-4459?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14215202#comment-14215202
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Alok Saldanha commented on SPARK-4459:
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I created a standalone gist to demonstrate the problem, please see
https://gist.github.com/alokito/40878fc25af21984463f
> JavaRDDLike.groupBy[K](f: JFunction[T, K]) may fail with typechecking errors
> ----------------------------------------------------------------------------
>
> Key: SPARK-4459
> URL: https://issues.apache.org/jira/browse/SPARK-4459
> Project: Spark
> Issue Type: Bug
> Components: Java API
> Affects Versions: 1.0.2, 1.1.0
> Reporter: Alok Saldanha
>
> I believe this issue is essentially the same as SPARK-668.
> Original error:
> {code}
> [ERROR]
> /Users/saldaal1/workspace/JavaSparkSimpleApp/src/main/java/SimpleApp.java:[29,105]
> no suitable method found for
> groupBy(org.apache.spark.api.java.function.Function<scala.Tuple2<java.lang.String,java.lang.Long>,java.lang.Long>)
> [ERROR] method
> org.apache.spark.api.java.JavaPairRDD.<K>groupBy(org.apache.spark.api.java.function.Function<scala.Tuple2<K,java.lang.Long>,K>)
> is not applicable
> [ERROR] (inferred type does not conform to equality constraint(s)
> {code}
> from core/src/main/scala/org/apache/spark/api/java/JavaRDDLike.scala
> {code}
> 211 /**
> 212 * Return an RDD of grouped elements. Each group consists of a key and
> a sequence of elements
> 213 * mapping to that key.
> 214 */
> 215 def groupBy[K](f: JFunction[T, K]): JavaPairRDD[K, JIterable[T]] = {
> 216 implicit val ctagK: ClassTag[K] = fakeClassTag
> 217 implicit val ctagV: ClassTag[JList[T]] = fakeClassTag
> 218 JavaPairRDD.fromRDD(groupByResultToJava(rdd.groupBy(f)(fakeClassTag)))
> 219 }
> {code}
> Then in core/src/main/scala/org/apache/spark/api/java/JavaPairRDD.scala:
> {code}
> 45 class JavaPairRDD[K, V](val rdd: RDD[(K, V)])
> 46 (implicit val kClassTag: ClassTag[K], implicit
> val vClassTag: ClassTag[V])
> 47 extends JavaRDDLike[(K, V), JavaPairRDD[K, V]] {
> {code}
> The problem is that the type parameter T in JavaRDDLike is Tuple2[K,V], which
> means the combined signature for groupBy in the JavaPairRDD is
> {code}
> groupBy[K](f: JFunction[Tuple2[K,V], K])
> {code}
> which imposes an unfortunate correlation between the Tuple2 and the return
> type of the grouping function, namely that the return type of the grouping
> function must be the same as the first type of the JavaPairRDD.
> If we compare the method signature to flatMap:
> {code}
> 105 /**
> 106 * Return a new RDD by first applying a function to all elements of
> this
> 107 * RDD, and then flattening the results.
> 108 */
> 109 def flatMap[U](f: FlatMapFunction[T, U]): JavaRDD[U] = {
> 110 import scala.collection.JavaConverters._
> 111 def fn = (x: T) => f.call(x).asScala
> 112 JavaRDD.fromRDD(rdd.flatMap(fn)(fakeClassTag[U]))(fakeClassTag[U])
> 113 }
> {code}
> we see there should be an easy fix by changing the type parameter of the
> groupBy function from K to U.
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